I am convincing my wife to leave Photoshop in favour for Pixelmator Pro. There is one thing though that she does frequently that I haven't been able to figure out myself.

In photoshop she starts to create a A4 document with 300 DPI. Then she crops a lot of pieces from other photographs and copy them to this A4 document so that she can print this A4 document on a photo printer.

I need to be able to copy for example a 3cmX4cm selection from another photograph so that I can paste them into the A4 document to fill upp the A4 document before printing?

You *could* change the canvas size to 3x4cm then export the image. You could create a 3x4 cm rectangle and use that as a guide to make a slice using the export for web functionality. There's probably a bunch of other ways you could export a 3x4cm image.

I'm going to try to persuade you not to.

Pixelmator Pro has some very powerful masking tools that let you put all the images you want on a page and mask them in place. To do this.
1. Create your A4 300 ppi image.
2. Create a 3x4cm rectangle (the shape tool has drop-down so that you can specify sizes in cm, inches etc).
3. Style the shape so that it has a solid fill with no border and place it where you want the image to be.
4. Add a photograph and place it in the layer above the rectangle.
5. Right-click on the photograph in the layers panel and select Create Clipping Mask.
6. Move, size and rotate the photograph to taste. The rectangle will provide the crop. The photograph will provide the image.
Repeat 2-6 until you have added and placed all the photographs you want.
If you have a layout you want to keep, delete all the photographs and save the image with the rectangles in place. You can then reuse this as a template and just add the new photographs to it.

Give it a shot, show it to your wife and see what you both think. Let me know how it goes?